1. Field of the Invention
This invention relates to a signal detecting system which detects an image signal by correlation dual sampling.
2. Description of the Related Art
Conventionally, there has been used in various fields a photoelectric conversion means such as a CCD which receives light and outputs a signal by converting the received light to a charge signal or a photomultiplier or an image read-out system which stores electric charges upon exposure to radiation and outputs a charge signal according to the stored electric charges. A charge amplifier which can be made in the form of an IC and is relatively low in noise has been used as a detector for detecting the charge signal output from the photoelectric conversion means or the image read-out system. The charge amplifier starts to store the charge signals upon switching to a storing mode while discharges the stored charge signals and outputs a charge signal according to the stored electric charges upon switching to a reset mode.
The switching to the storing mode in the charge amplifier is carried out by switching a reset switch from an ON state to an OFF state. kTC noise is generated in response to switching of the reset switch and the generated noise is included in the charge signal which is a component of the signal. In order to avoid the influence of the kTC noise, the correlation dual sampling is carried out. In the correlation dual sampling, the difference between the charge signals respectively output a predetermined baseline sampling time after the signal storing circuit is switched to the storing mode and immediately before the signal storing circuit is switched to the reset mode is taken and by making the difference a signal component, the influence of the kTC noise can be avoided.
In circuits where the correlation dual sampling is carried out, a low-pass filter is provided after the signal storing circuit in order to reduce the high frequency noise in the charge signal output from the signal storing circuit. See, for instance, Japanese Unexamined Patent Publication No. 2004-147255.
When the image information is to be read at high speed by shortening the time required to read one pixel (one cycle), so-called a field-slew period is also shortened. At this time, there is problem that fluctuation of the reference level signal does not converge and the reset noise is included in the reference level signal, whereby the dynamic range deteriorates.